The present invention relates to a content distributing system, an encrypting apparatus, a content offering apparatus, a content reproducing apparatus, a license information offering apparatus, an encrypting method, a content offering method, a content reproducing method, a license information offering method, an information processing program, and a storage medium. More particularly, the invention relates to techniques for reinforcing the capability of concealing content by dynamically generating an encryption key for each of the objects constituting the content.
In recent years, varieties of digital content have been distributed in growing quantities over the Internet. Digital content (simply called content hereunder) is constituted illustratively by text information, still pictures, moving pictures, video games, and other publications converted into electronic data. As such, content is reproduced by personal computers or by dedicated reproducing apparatuses for use.
Content is delivered to users illustratively over networks. In today's electronized society, this mode of content delivery poses a big challenge: how to let legitimate users alone make use of content and to keep unauthorized users from illicitly uses in order to protect copyrights.
Conventional methods for enabling the legitimate user to use content typically involve encrypting content using a common key while allowing the user to acquire this common key so that the authorized user alone can decrypt the content using the acquired key. One such method is disclosed illustratively in Japanese Patent Laid-open No. 2002-359616.
The disclosed technique requires clients to acquire licenses in order to reproduce content. A header attached to content is arranged to include information for specifying a relevant license. Only if a client is in possession of the license specified by the information is it possible for the client to decrypt and reproduce the content in question.
Conventionally, all objects included in content were encrypted using the same encryption key. Once the encryption key was identified, it was possible to decrypt all objects. For example, even if a block chaining scheme such as CBC mode of DES (Data Encryption Standard) is utilized, once an initial vector and a common key are identified, all data can eventually be decrypted. Block chaining is a method that involves encrypting the first block, then utilizing an outcome of the encryption of the first block in order to encrypt the next block, and repeating the encrypting process in chained fashion.
Preparing a large number of encryption keys in advance could conceivably boost the capability of data concealment. This measure, however, has the disadvantage of increasing content and key sizes because of the numerous keys to be prepared for encryption.
It is therefore an object of the present invention to provide an encrypting apparatus and a reproducing apparatus, as well as related methods and other resources for encrypting content in a manner enhancing the level of content concealment while allowing the encrypted content to be decrypted efficiently.